1. Technical Field
Aspects of this document relate generally to improved methods and systems for reducing fuel consumption and improving aerodynamic efficiency and performance of vehicles, especially regarding boundary layer reduction and enhanced laminar flow systems. Particular implementations also include parasitic drag induced boundary layer reduction systems.
2. Background Art
Laminar flow occurs when a fluid flows in parallel layers, with no disruption between the layers. In fluid dynamics, laminar flow is a flow characterized by high momentum diffusion and low momentum convection. For the purpose of understanding the following descriptions, non-laminar flow, sometimes referred to as turbulent flow, results when laminar flow is compromised by one or more factors as further described below.
In factoring flow conditions leading to laminar or turbulent flow, an important parameter in the equations is the Reynolds number. In fluid mechanics, the Reynolds number is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions. The Reynolds number upon which laminar flows become turbulent is dependent upon the flow geometry. When the Reynolds number is much less than 1, Creeping Motion or Stokes Flow occurs (an extreme example of laminar flow where viscous/friction effects are much greater than inertial forces).
At high Reynolds numbers it is desirable to have a laminar boundary layer. This results in an effectively lower skin friction due to the interaction between the characteristic velocity profile of laminar flow and the primary flow. However, as the flow develops along the body, the boundary layer increases and becomes less stable, eventually becoming turbulent. This process is known as boundary layer transition.
At lower Reynolds numbers, it is relatively easy to maintain laminar flow, with resulting low skin friction. However, the same velocity profile that gives the laminar boundary layer its low skin friction also introduces adverse pressure gradients known as pressure drag. Therefore, as the pressure begins to recover over the rear part of the wing chord, a laminar boundary layer will tend to separate from the surface. Such flow separation causes a significant increase in pressure drag (hereinafter parasitic drag), since it greatly increases the effective size of the body section.
Specifically relating to the flow of air over an airplane wing, the boundary layer is a relatively thin ‘sheet’ of air lying over the surface of the wing (and other surfaces of the airplane). Because air has viscosity (friction interaction with adjacent particles moving at different velocities), this layer of air tends to adhere to the wing. As the wing moves forward through the air, the boundary layer at first flows smoothly over the streamlined shape of the airfoil and the boundary layer is a laminar layer. However, as the speed of the wing increases, the boundary layer breaks away from the surface and creates an increasing low-pressure region immediately behind the airfoil (sometimes referred to as flow separation). This low-pressure region results in increased overall drag (principally parasitic drag). Furthermore, that separation creates a boundary layer effect, where laminar flow is compromised and more turbulent flow results, decreasing the efficiency of the wing's ‘lift geometry’ and performance while also increasing drag. Those inefficiencies lead to high fuel consumption and limit the performance of the aircraft. Therefore, it is desirable to control or reduce the boundary layer.
Attempts have been made over the years to delay the onset of flow separation by careful attention to design geometry, smoothing of surfaces and other passive technologies such as vortex generators developed to reduce various factors to non-laminar flow, but those efforts have resulted in only minimal improvements.
In concept, a significant way of improving airflow by reducing the boundary layer includes boundary layer suction, an approach by which an ‘air pump’ is used to extract the boundary layer from the wing's top surface, thereby improving the airflow and reducing drag. However, the systems that have been used to produce the suction are very complicated, heavy, and prone to high rates of failure (from contamination and other factors), and usually require an additional engine (or additional work for existing engines) to act as or power the necessary air pump. Moreover, when these systems (commonly referred to as active) fail, the aircraft's flight performance can deteriorate significantly (including to critically dangerous levels).